Damage dealt to a creature with indestructible remains marked on that creature. If Erebos’s Titan has lethal damage marked on it and it loses indestructible (perhaps because your opponent controlled no creatures and then gained control of one), Erebos’s Titan will be destroyed.

6/22/2015

The last ability triggers only if Erebos’s Titan is in your graveyard.

6/22/2015

In a multiplayer game, the last ability won’t trigger when a player with a creature card in their graveyard leaves the game.